Well it's clear that a large proportion of Ukrainians prefer to speak Russian over Ukrainian. What is the problem with having two official languages and double signs? Who is against that, and why?

That way everybody can speak the language they prefer. If the Russian speakers feel discriminated against, they will become disloyal to the Ukrainian state and perhaps identify more with Russia/Russians.

Anyway, that kind of behaviour in the parliament is disgraceful and it is not good for Ukraine's image. I don't think it's the first time it happened either.